Calendar timezone component

Added a `Timezone` component ([documentation](http://tools.ietf.org/html/rfc2445#section-4.6.5)) that can be added directly to a calendar object for grouping timezone properties on the root calendar object (saves adding to each event).

Also included a cheeky `__toString()` method on components for super-lazy rendering:

```php
echo new Calendar('prod-id');
```